Process and equipment for plasmid purfication

ABSTRACT

A scalable alkaline lysis process, including procedures and devices for the isolation of large quantities (grams and kilograms) of plasmid DNA from recombinant  E. coli  cells. Effective, controllable, and economical operation, and consistent low level of host chromosomal DNA in the final plasmid product. Involves a series of new unit operations and devices for cell resuspension, cell lysis, and neutralization.

RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.09/774,284, filed Jan. 29, 2001 and published Oct. 25, 2001 as2001/0034435, which claimed priority to U.S. patent application Ser. No.08/887,673, filed Jul. 3, 1997, now abandoned, which in turn, claimedpriority to U.S. Provisional Patent Application Ser. No. 60/022,157,filed Jul. 19, 1996. Both applications are hereby incorporated byreference as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to the field of plasmid purification andis particularly useful in the field of gene therapy.

BACKGROUND OF THE INVENTION

The following description of the background of the invention is providedto aid in understanding the claimed invention, but it is not admitted toconstitute or describe prior art to the claimed invention and should inno way be construed as limiting the claimed invention.

The traditional alkaline lysis process for isolation of plasmid DNA frombacterial cells is described in Bimboim, H. C. and J. Doly, NucleicAcids Res. 7:1513, 1979 and is also described in J. F. Burke, NucleicAcids Res. 8:2989, 1981. It makes use of several laboratory-scaleapparatuses and manual operations which are not suitable for large-scalemanufacturing. In addition, the traditional alkaline lysis process isnot suitable for making pharmaceutical grade material for human use,since inconsistent process performance may result in an unacceptablelevel of sheared host chromosomal DNA in the final plasmid product. Thedifficulty of theoretical study of some of the critical parameters formanual operations, like shear force, also make the traditional processunlikely to become a large scale manufacturing process.

Such traditional processes consist of three stages. The first stage iscell resuspension, which normally utilizes manual stirring or a magneticstirrer, and a homogenizer or impeller mixer to resuspend cells in theresuspension buffer. Manual stirring and magnetic stirring are onlyappropriate for laboratory scale preparations.

The second stage is cell lysis. It is desirable to minimize shear ofhost chromosomal DNA at this stage, because it is difficult to removesheared chromosomal DNA in the downstream purification process (due tothe similarity in properties between sheared chromosomal DNA and plasmidDNA). The lysis is normally carried out by manual swirling or magneticstirring in order to mix the resuspended cells with lysis solution(consisting of diluted alkali (base) and detergents); then holding themixture at room temperature (20-25 degrees Celsius) or on ice for aperiod of time, such as 5 minutes, to complete lysis. As noted above,manual swirling and magnetic stirring are not scalable. In addition, itis difficult to optimize the process and obtain consistent processperformance due to the number of operation parameters, includingoperator to operator variability.

The third stage is neutralization and precipitation of hostcontaminants. Lysate from the second stage is normally mixed with a coldneutralization solution by gentle swirling or magnetic stirring toacidify the lysate before setting in ice for 10-30 minutes to facilitatethe denaturation and precipitation of high molecular weight chromosomalDNA, host proteins, and other host molecules. Again, both manualswirling and magnetic, stirring are not scalable. An ice bath is notconvenient if a long holding time is desired in a large scale process,because a large quantity of water must be removed and a large quantityof ice/or ice-NaCl mixture needs to be added periodically to maintain asteady temperature.

Once the plasmid DNA is extracted from the lysed cells its purificationhas become a routine and important procedure for the molecularbiologist. However, the scale for these purifications, often referred toas “mini-preps”, is usually less than about 1 milligram of plasmid DNA.These small scale preps isolate plasmid DNA from the supernatant oflysed bacterial cells using a variety of techniques, such as ethanolprecipitation. For slightly larger scale preparations, the primarytechniques employed use cesium chloride centrifugation, binding andeluting to silica resins (in the presence of chaotropic salts) orbinding and eluting with various anionic chromatography resins. Inaddition, other techniques are sometimes used in combination with theresins mentioned, e.g., PEG and/or alcohol precipitation, RNasetreatment, and phenol/chloroform extraction. There are also some plasmidpurifications performed using analytical HPLC, in particular reversephase HPLC to separate different plasmid forms using organic solventsystems.

Reverse phase chromatography (“RPC”) is generally practiced by bindingcompounds of interest to a chromatography support in an aqueous solutionand eluting with increasing amounts of an organic solvent, such asacetonitrile or alcohol. This approach has been used by a number ofinvestigators to separate DNA, especially oligonucleotides and small(less than 1,000 base pair) restriction fragments. It has also been usedto separate open circular and supercoiled plasmids. However, RPCsolvents present volatility, fire, health and waste disposal risks. Inaddition, RPC of nucleic acids frequently requires the use of anion-pairing agent (e.g., triethylammonium acetate or TEAA) which can bedifficult to remove from the DNA and can be toxic to cultured cells.

The “mini-prep” procedures described above were designed for purifyingsmall amounts of plasmid DNA and in general they have not been suitablefor large-scale, high throughput purification processes. Large scalepurification of plasmid DNA may magnify the contaminants in the finalpurifications in the final preparations, which usually go undetected inmini-preps. Anion-exchange chromatography as a single chromatographystep is unlikely to remove enough of the contaminants which would benecessary for a therapeutic product. Thus, additional processes shouldprobably be included to increase the purity of the plasmid DNA. Inaddition, it would be useful to have a scalable process which could alsoresolve various plasmid forms.

Two contaminants which may be particularly troublesome are RNA andchromosomal DNA. Many mini-prep procedures attempt to remove RNA usingone or several RNase enzymes which degrade the RNA to ribonucleotidesand small oligoribonucleotides. These can then be separated from theplasmid DNA using any of a variety of techniques, including alcoholprecipitation, size exclusion chromatography, anion exchangechromatography, etc. However, the use of RNase is undesirable in largescale (equal to or greater than 50-100 mg of plasmid) purification.RNase is an expensive material that is generally not reused. Large scaleRNase reactions can be difficult to perform in batch mode withappropriate control of time, temperature and other reaction conditions.Also, RNase is typically isolated from bovine pancreas. As such, it is apossible source of mammalian pathogens, especially retrovirus and bovinespongiform encephalopathy (BSE). Use of such materials in makingplasmids for human use presents significant safety and regulatoryissues.

Another approach to removal of RNA involves differential precipitationswhereby plasmid DNA is precipitated while RNA remains in solution orvice versa. An example is described in WO 95/21250 in which polyethyleneglycol (PEG) is used to precipitate RNA from a solution containing bothRNA and plasmid. Similar techniques have been described. A disadvantageof these techniques as typically practiced is that the plasmid is firstpartially purified from the lysate by removing solid debris,precipitated proteins and other solids, and optionally by alcoholprecipitation of the nucleic acids (including RNA and plasmid).Differential precipitation is then applied to the partially purifiedmixture of RNA and plasmid. This approach to differential precipitationhas required multiple steps, increasing the time effort and complexityof the process, and introducing more opportunities for loss of plasmid.

Removing chromosomal DNA derived from the bacterial host is also achallenging task in plasmid purification. In a typical “mini-prep”,chromosomal DNA is removed primarily during the lysis and neutralizationsteps. Large fragments of chromosomal DNA are bound to proteins andmembrane fragments, and are carried into the precipitate during theneutralization step. However, it is well known that if the chromosomalDNA is sheared to smaller size (≦about 10 kb), it is not efficientlyprecipitated and contaminates the plasmid DNA. Thus, it has been assumedthat during mini preps, one must avoid vigorous mixing and shearing ofthe lysate. This is difficult to achieve at larger scales due to theknown difficulties of mixing large volumes of liquid. Thus, it isdesirable to have a way to separate plasmid DNA from chromosomal DNAsubsequent to lysis and neutralization.

One approach to this is CsCl/ethidium bromide density gradients. Theseare very effective for small amounts of plasmid (≦1 mg) that are notintended for human use. They are not generally suitable for scaling upto over 100 mg lots due to the high cost of necessary equipment. Theyare also not generally suitable for producing plasmid for human usebecause ethidium bromide binds tightly to DNA, is difficult to removequantitatively, and is a known mutagen and suspected carcinogen.

Thus, there are several clear needs for large scale plasmidpurification. These include a method to mix large volumes of lysate withlow shear, a method to precipitate RNA directly from a lysate withoutprior additional purification, and a method to separate shearedchromosomal DNA from plasmid DNA.

Other methods for purification of larger amounts of plasmid DNA are notideal and leave significant room for improvement. For example, themethod described in WO 95/21250, published Aug. 10, 1995 involvesmultiple precipitations, plasmid precipitations, low capacity sizeexclusions, and requires flammable alcohols. Similarly, the methoddescribed in WO 96/02658, published Feb. 1, 1996, requires lysozyme, mayrequire RNase, and requires flammable alcohols.

SUMMARY OF THE INVENTION

The present invention relates to a scalable alkaline lysis process,including procedures and devices for the isolation of large quantities(grams and kilograms) of plasmid DNA from recombinant E. coli cells. Thepresent invention provides effective, controllable, and economicaloperation, and consistent low level of host chromosomal DNA in the finalplasmid product. These attributes, and the details which follow, clearlyprovide advantages over the traditional alkaline lysis process and themethods proposed by others for commercial scale procedures.

The present invention exploits a set of devices and procedures capableof providing fast and efficient cell resuspension in a semi-continuousmode. All device's design and operation parameters can be wellcharacterized and optimized through empirical tests in both scale-downand scale-up processes. Both mixing quality and shear force can be wellcontrolled to maximize mixing efficiency and to minimize possible damageto cells.

The present invention also provides a set of devices and procedurescapable of providing efficient and gentle mixing and cell lysis in acontinuous flow mode. The device's design (e.g. device dimension) andoperation parameters (e.g. flow rate and residence time) were wellcharacterized through empirical tests and theoretical calculations andmodeling to maximize lysis efficiency and minimize shearing ofchromosomal DNA. Surprisingly, by performing alkaline lysis using a highconcentration of unbuffered salt, not only is denatured chromosomal DNA,protein, and cellular debris trapped in a precipitable salt/detergentcomplex, but a significant portion of RNA is also precipitated, therebyeliminating the need for an RNase treatment step and providing asignificant advantage.

In addition, the present invention provides a set of devices andprocedures capable of providing fast chilling and efficient and gentlemixing to denature and precipitate chromosomal DNA, protein, and RNA.The devices' selection and design and operation parameters were wellcharacterized through scale-up-empirical testing.

The invention relies, in part on the use of anion exchangechromatography, preferably with a Fractogel EMD 650S TMAE(S) resin witha 20-40 micron size. Other tentacle resins or means for making bindingsites more available to larger molecules like plasmids can also be used.The tentacles are preferably 15 to 50 units in length and have anaverage of 18 charged groups covalently bound to each tentacle. TMAEFractogel 650S is a tentacle ion exchanger having trimethylaminoethylfunctional groups (TMAE) covalently attached to hydroxyl groups of asynthetic methacrylate based polymeric resin backbone.

The use of such resins: (1) provides a high plasmid DNA binding capacity(about 3 mg/mL, preferably about 1.5 mg/mL); (2) allows for efficientremoval of proteins, RNA, low molecular weight molecules and probablysome chromosomal DNA and some open circle plasmid DNA; and (3) providesa means for enriching the supercoiled plasmid DNA above about 80% usinga step gradient. Supercoiled plasmid binds tighter to the Fractogelresin (high affinity sites) allowing the remaining RNA and some opencircle plasmid to be removed.

The invention also relies in part on the use of hydrophobic interactionchromatography, which is used to separate plasmid DNA from E. colichromosomal DNA and RNA and may also be used to separate open circularplasmid DNA from supercoiled DNA. Overall, HIC is a powerful techniquefor plasmid DNA purification. This disclosure reveals the surprising andunexpected value that hydrophobic interaction chromatography (HIC) has,especially when used in conjunction with anion-exchange chromatography,for large-scale plasmid DNA purification. Particularly surprising is theability of HIC to resolve the supercoil form of a plasmid from therelaxed open circle form. Supercoiled DNA may be easier to formulate andwith certain formulations supercoiled plasmid may have higher expressionlevels in vivo (e.g., about 10 times greater expression in certainsystems). Another surprising and important fact is that removal ofchromosomal DNA, denatured plasmids, RNA, and endotoxin from the plasmidDNA forms can also be achieved.

Thus, in one aspect the invention provides a process for isolating alarge quantity (e.g. gram or kg amounts) of plasmid DNA. The methodinvolves the steps of: (a) lysing cells containing the plasmid DNA witha lysis agent, thereby forming a lysate; (b) treating the lysate with ahigh salt agent that preferably is capable of forming a precipitablecomplex with non-plasmid DNA cellular components contained in thelysate, thereby forming a treated solution; and (c) purifying thetreated solution to provide isolated plasmid DNA.

The phrase “plasmid DNA” is meant to include all forms of plasmid DNA,such as supercoiled plasmid DNA (type I), nicked circle plasmid DNA(type (II), and linearized plasmid DNA (type III), as well as denaturedplasmid DNA. The plasmids may include any of a wide variety of vectors,origins of replication, and genetic elements (such as selectable genes,polylinkers, promoters, enhancers, therapeutic genes, leader peptidesequences, introns, polyadenylations signals, etc.) known to thoseskilled in the art. Genes encoding diverse proteins (includingtherapeutic proteins such as IL-2, IL-12 and the like) may be insertedinto the plasmid, and the genes may constitute, for example, genomicDNA, cDNA, synthetic DNA, and polynucleotide and oligonucleotidesequences.

By “lysis agent” is meant any agent capable of breaking open a cellcontaining plasmid DNA and thereby releasing the contents of the cell.The lysis agent preferably is alkali or basic, i.e., it has a pH above7.0, preferably about 12 to 13. Many suitable agents are known to thosein the art, for example, a solution containing 0.2N sodium hydroxide in1% SDS. Other concentrations of sodium hydroxide in SDS may also beused. Other detergents (tween, np40, sarkosyl, etc.) such as non-ionicdetergents (e.g., triton X) and lysozyme plus heat treatment may allalso be used.

By “high salt agent” is meant a substance that is capable ofprecipitating out a significant portion (preferably at least 25%, morepreferably at least 50%, most preferably at least 75% or 90%) of any RNAmolecules released from the cells. The high salt agent preferably hasone or more salts at a pH above approximately 5.5, for example a mixtureof IM potassium acetate and 7M ammonium acetate at a pH between 7.0 and9.0. It may be possible to replace the potassium acetate with apotassium agent or ion and/or KCl, NaCl, or NH₄OAc. The lysate ispreferably treated with the high salt agent for at least six hours atapproximately four degrees Celsius.

In preferred embodiments, the process does not involve the use of RNase,the process yields isolated plasmid DNA that is pharmaceutical-gradeplasmid DNA suitable for administration to humans, and at least 100milligrams of the isolated plasmid DNA is obtained, more preferably atleast 1 g, most preferably at least 10 g, 100 g or 1,000 g (1 kg). Inother preferred embodiments, the invention provides a process forisolating plasmid DNA involving the steps of: (a) resuspending cells inapproximately 50 mM of Tris-HCl buffer at a pH of about 8.0 andapproximately 10 mM EDTA(Na)₂; (b) lysing cells containing the plasmidDNA with a lysis agent comprising an approximately equal volume of 0.2Nsodium hydroxide in 1% SDS, thereby forming a lysate; (c) treating thelysate with a high salt agent that comprises a mixture of 1M potassiumacetate and 7M ammonium acetate at a pH between 7.0 and 9.0, preferablycapable of forming a precipitable complex with non-plasmid DNA cellularcomponents contained in the lysate, thereby forming a treated solution;and (d) purifying the treated solution to provide isolated plasmid DNA.Step (d) may involve subsequently isolating from the supernatant of thecellular lysate a sample of highly purified supercoiled plasmid DNA.

In view of the above, it can be seen that, among other things, theinvention provides an improved process for isolating plasmid DNA fromalkaline lysates of a cell containing the plasmid DNA by treating thelysate with a high salt agent, preferably capable of forming aprecipitable complex with non-plasmid DNA cellular components containedin the lysate.

In another aspect, the invention provides a process for isolatingplasmid DNA involving the steps of: (a) lysing cells containing theplasmid DNA with a lysis agent, thereby forming a lysate; and (b)purifying the lysate with anion exchange chromatography using a stepgradient, thereby producing isolated plasmid DNA enriched with at least80% supercoiled plasmid DNA.

In preferred embodiments the anion exchange chromatography is performedwith a resin having a particle size of 20-40 microns, the anion exchangechromatography has a plasmid DNA binding capacity of about 1.5 mg ofplasmid per mL of resin, more preferably a plasmid DNA binding capacityof about 3 (or more) mg of plasmid per mL of resin, and the anionexchange chromatography is performed with a Fractogel EMD TMAE (650-S)resin. Thus, the invention provides an improved process for isolatingplasmid DNA from lysate of cells containing the plasmid DNA by purifyingthe lysate with anion exchange chromatography using a step gradient,thereby producing isolated plasmid DNA enriched with at least 80%supercoiled plasmid DNA.

In another aspect, the invention features a process for isolatingplasmid DNA involving the steps of: (a) lysing cells containing theplasmid DNA with a lysis agent, thereby forming a lysate; and (b) usinghydrophobic interaction chromatography (HIC) to purify the lysate,thereby providing isolated plasmid DNA. Typically the lysate is notapplied directly to the HIC column directly, but is first sent throughan anion exchange chromatography resin, such as the Fractogel TMAE resindiscussed herein.

In preferred embodiments the hydrophobic interaction chromatography isperformed with at least 1.6M ammonium sulfate (at 2.OM ammonium sulfatethe supercoiled plasmid binds to the HIC column), Tris, and EDTA, andthe hydrophobic interaction chromatography is performed with an OctylSepharose 4 FF resin. This is a flow through column that allowssupercoiled plasmid to flow through the column while RNA, andchromosomal DNA endotoxin and denatured DNA binds to the resin. Thecolumn is moderately hydrophobic due to octyl alkyl chains as its ligandon agarose beads. Other resins having similar properties such as phenylsepharose, butyl sepharose, and others, may also be used.

In other preferred embodiments the plasmid DNA is not precipitated andthe process involves no linear gradients and uses no organic solvents,the isolated plasmid DNA is substantially free of endotoxins (forexample, 10 to 100 Endotoxin Units per mg of plasmid DNA as measuredwith a Limulus Amebocyte Lysate Assay) and host cell chromosomal DNA(for example, about 1% to 3% as measured by LCR), the plasmid DNA is notexposed to acidic pH or elevated temperatures (e.g., about pH 8 and 22°C.), the isolated plasmid DNA is produced in a yield of at least 60%(preferably 70%, more preferably 80%, most preferably 90%), thehydrophobic interaction chromatography is performed in an aqueoussolution containing a high concentration of salt, such as 1.6M ammoniumsulfate, and the cells are recombinant E. coli cells. Thus, theinvention features an improved process for isolating plasmid DNA from alysate of a cell containing the plasmid DNA by using hydrophobicinteraction chromatography to purify the lysate, thereby providingisolated plasmid DNA. Preferably the lysate is first partially purifiedwith anion exchange chromatography, for example with a TMAE resin, andthen the anion exchange chromatography pool, (for example, a TMAE pool)is further purified with HIC.

In yet another aspect, the invention provides a device for isolatingplasmid DNA from cells containing the plasmid DNA, comprising: (a) ameans for providing cell resuspension in a semi-continuous mode; (b) ameans for providing mixing and cell lysis in a continuous flow mode; and(c) a means for providing chilling and mixing to denature andprecipitate chromosomal DNA, protein, and RNA.

In preferred embodiments, the means for providing efficient and gentlemixing and cell lysis in a continuous flow mode comprises an impellermixer, an in-line static mixer, and/or a lysis coil. The devicepreferably has a means for performing hydrophobic interactionchromatography and the means for providing fast chilling and efficientand gentle mixing to denature and precipitate chromosomal DNA, protein,and RNA preferably comprises a chilled jacketed tank.

Other and further objects, features, and advantages will be apparentfrom the following description of the drawings and the presentlypreferred embodiments of the invention, as well as the examples providedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process diagram including resuspension and lysis(in line mixer), and neutralization (impeller mixer).

FIG. 2 is a flow chart for Process I.

FIG. 3 is a flow chart for Process II.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention identified here is a novel, scalable, and alternativealkaline lysis process for large scale plasmid DNA isolation. Thepresent invention involves a series of new unit operations and devicesfor cell resuspension, cell lysis, and neutralization. The invention canprovide controllable and efficient operation and consistent performanceand is economical. Described below are various preferred methods forperforming the invention as well as a detailed description of thedevices and certain aspects of the invention.

The most preferred embodiments are described immediately below, followedby a discussion of several of the steps and devices that may be usefulin practicing the invention.

One of the preferred embodiments involves the use of a continuous flowdevice with in-line mixing for resuspending bacterial cells, lysing withalkali and detergent, and neutralizing the lysate. This has severaladvantages (relative to batch operation in multiple bottles or largetanks), including: (1) better cell resuspension and mixing of solutions;(2) better control of lysis times; and (3) lower shear, proving lesschromosomal DNA contamination (very significant).

A second preferred embodiment involves the use of a modified, non-acidicneutralization solution that can be added to an alkaline/detergentlysate to precipitate RNA as well as chromosomal DNA, protein, lipids,etc. from the lysate. Some of the advantages (relative to 3M potassiumacetate, pH ˜5) are: (1) removes majority of RNA (especially highmolecular weight) prior to column chromatography (very significant); (2)eliminates exposure of plasmid DNA to damaging acid; (3) eliminates needto partially purify plasmid prior to RNA precipitation.

A third preferred embodiment involves the use of Fractogel EMD TMAEtentacle ion exchange resin to separate supercoiled plasmid from opencircular plasmid and from protein and low molecular weight RNA. Here,the advantages (relative to Sepharose Q or other low pressure ionexchange resins) include: (1) higher capacity (1.5 mg plasmid/mL resin);(2) high flow rate; (3) able to separate supercoiled and open or nickedcircular plasmid using a simple, convenient, scalable salt step gradient(very significant).

A fourth preferred embodiment involves the use of hydrophobicinteraction chromatography (HIC) to separate plasmid DNA fromchromosomal DNA, RNA, protein, endotoxin, and denatured plasmid DNA. Theadvantages of this embodiment include: (1) removes E. coli DNA (verysignificant); (2) removes residual RNA, endotoxin, and denatured plasmidDNA; (3) optionally separates supercoiled from nicked or open circularplasmid.

The fifth preferred embodiment is an integrated processes employing someor all of the above (Process I, Process II, and potential variants).

Alkaline Lysis

Conventional alkaline lysis techniques involve the use of 3M potassiumacetate at a pH of about 5.5. A significant improvement presented hereinis the use of high concentrations of unbuffered salt. (A number ofmonovalent salts are suitable and some divalent salts will also haveacceptable solubility properties.) For example, by adding a volume of anunbuffered solution of 1M potassium acetate and 7M ammonium acetate andletting the mixture stand for at least 2 (preferably at least 6) hoursat 4 degrees Celsius a surprising and unexpected result is achieved.Namely, not only is denatured chromosomal DNA, protein, and cellulardebris trapped in a precipitable salt/detergent complex, but asignificant portion of RNA is also precipitated, allowing for theelimination of an RNase treatment step. The potassium acetate preferablyis present in an excess in order to aid in the precipitation the SDS DNAcomplex. Furthermore, the RNA that remains in solution is typically lowmolecular weight and can be separated from plasmid DNA by subsequentchromotographic steps (e.g., anion exchange, HIC, size exclusion, etc.).At lower dilutions to the TMAE load (1.5 vs. 3 or 4 volumes), themajority of RNA that remains flows through the TMAE column.

Anion Exchange Chromatography

The present invention makes use of anion exchange chromatography,preferably with a resin such as Fractogel EMD TMAE(s) 650-S having a20-40 micron particle size. This advantageously provides a high DNAbinding capacity of about 3 mg/mL, preferably about 1.5 mg/mL and allowsfor the efficient removal of proteins, RNA, low molecular weightmolecules, and probably some chromosomal DNA. Surprisingly, thisinvention allows for the enrichment of supercoiled form of plasmid DNAby over 80% using a step gradient. Enrichment of supercoiled plasmid isachieved by removal of more open circular plasmid than is achieved inthe preferred embodiment of Process I outlined in FIG. 2. The percentageof enrichment may be measured with a 1% reliant gel (FMC), 0.8% agarosegel electrophoresis, and may also be measured using an ion exchange HPLCassay using, for example, a DEAE column.

Hydrophobic Interaction Chromatography

Hydrophobic interaction chromatography (“HIC”) is generally practiced bybinding compounds of interest in an aqueous solution containing highconcentrations of salt (e.g. ammonium sulfate). Elution is accomplishedby lowering the salt concentration. No volatile or flammable organicsolvents are used, and no organic modifiers like TEAA are generallyrequired.

There are a variety of HIC resins which are commercially available,differing in both backbone and functional chemistries. In general, theycan be made to work for plasmid DNA purification, however there are somewhich are preferred. The HIC step is a flow through step, i.e., thesupercoiled and open circular plasmid flow through the column while RNA,chromosomal DNA, denatured plasmid DNA and endotoxins are retained onthe column.

The HIC resins are initially equilibrated in high ionic strengthmultivalent salts such as ammonium sulfate. These conditions enhancehydrophobic interaction and thus retention of various biomolecules. Byreducing the salt concentration the hydrophobic interaction is lessenedand the biomolecules begin to elute from the resin depending on theirhydrophobicity. In the case of plasmid DNA, it has been observed thatthe relaxed open circle form has very little interaction with HIC resinsin the presence of high salt. While under these conditions thesupercoiled form does interact and is retained by the resin leading to aresolution of the forms. As the salt concentration is lowered, thesupercoiled form is eluted, while the RNA, chromosomal DNA and denaturedplasmid DNA remain bound. Thus, the supercoiled plasmid may be purifiedfrom undesired plasmid forms (open circular, denatured) and fromcontaminants (chromosomal DNA, RNA, protein, endotoxin) using HIC. Thisis an unexpected result.

The interaction of biomolecules with the HIC resins can also be modifiedas a function of the multivalent salt selected. It has been observedthat in general sodium citrate and potassium phosphate salts canincrease hydrophobic interaction of plasmid DNA greater than ammoniumsulfate. Thus, by optimizing the selection of resin and salt conditions,one can affect the quality of purification for a given plasmidconstruct.

Devices

All devices described in this invention were built to large scale(process capacity is greater than 2 kg of wet cell paste) to demonstratethe feasibility and effectiveness of the invented devices and operationprocedures, and the concept of continuous-flow-through resuspension,lysis, and neutralization. A polypropylene or stainless steel conicaltank with a bottom three-way valve was connected to two sets ofplatinum-cured silicone tubing. Those skilled in the art will recognizethat many other types of containers and/or connections may besubstituted without substantially changing the function, mode ofoperation or result obtained the overall device. One set of tubingcontained one unit of ½″ in-line static mixer and two units of ⅜″in-line static mixers. These in-line mixers are for resuspending cellsand homogenizing cell concentration. Other sizes and numbers of mixersmay also suitably be used. The other set of tubing is used to transferresuspended cells to contact lysis solution.

A MasterFlex peristaltic transfer pump was used for resuspension andhomogenization. Other suitable (exp-low shear) pumps are available andmay be substituted without materially altering the overall device. A 43ft×1″ OD stainless steel tubing was coiled with approximately 2′diameter and a 6″ pitch and used as the lysis coils. Those skilled inthe art will recognize that the exact length, diameter, and pitch may bevaried by at least 10% and perhaps even over 50% without departing fromfunctional lysis goal served by the coils. The bottom of the lysis coilswas connected to a 1″ in-line static mixer (multiple mixers may be used)which is used to mix resuspended cells and lysis solution. A MasterFlexperistaltic pump with two stacked pump heads is used to transferresuspended cells and lysis solution to the 1′ in-line static mixer formixing. As noted above, other pumps and mixers may also be used.

The holding tank for precipitation is a 150 L stainless steel jacketedtank. Other suitable containers will preferably have a volume of atleast 100-200 L or more. The tank jacket was connected to arecirculating chiller which has the ability to chill the coolant to −15°C. Other chilling means may be substituted and preferably have theability to achieve at least 10 degrees Celsius, more preferably zerodegrees or lower. Since two approaches were adopted to mix lysate andneutralization solution, two sets of operations and devices were used toaccomplish the mixing of lysate and neutralization solution. In thefirst approach, the effluent of the lysis coils went directly into thechilled tank. An impeller mixer was used to mix lysed cells andneutralization solution inside the tank. The second approach involves a1″ or 2″ (or other conveniently sized) in-line static mixer. Lysed cellsand neutralization solution were pumped into the in-line static mixersimultaneously (one skilled in the art will recognize that cells orlysis solution could be pumped first, followed by the other), and theeffluent of the mixer was directed into chilled tank for precipitation.

It is possible to run the EM Merck Fractogel EMD 650(s) TMAE column in adisplacement mode where super coiled plasmid displaces open circularplasmid off the resin, thereby enriching for super coiled plasmid. Thecolumn is equilibrated in 0.5M NaCl, TE buffer, the load (neutralizedlysis supernatant process 2) is diluted with 1.5 volumes WFI then loadedonto the column. The column is loaded at 3.0 mg/ml resin. The lastquarter of the flow through of the load to the TMAE column isrecirculated over the column to allow SC plasmid to compete off opencircular plasmid. The column is then washed to baseline with 0.5M NaCl,TE after the recirculation has finished. The plasmid is then eluted with1.9M (NH₄)₂SO₄.

Another resin from EM that displays similar properties. The resin is aFractogel (R) EMD TMAE Hicap (M). This is a high capacity resin withbetter flow characteristics than the (S) TMAE resin. This Hicap resinmay allow binding plasmid between 3 and 5 mg/ml of resin and allow forthe enrichment of SC plasmid at a very high level.

EXAMPLES

The following examples are solely for illustrative purposes and are notmeant to limit the scope of the invention.

Example 1 Process I

The step-by-step methodology of Process I is outlined in FIG. 2.

Step 1 involves fermentation, cell harvesting, and washing. These arewell known procedures that may be carried out in wide variety of waysknown to those skilled in the art.

Step 2 involves alkaline lysis and neutralization. This step extractsplasmid, chromosomal DNA, RNA and protein. As explained in detail above,use of a high concentration of buffered salts is preferable.

Step 3 involves centrifugation and filtration. These also are well knowntechniques that may be performed in a variety of ways known to thoseskilled in the art.

Step 4 is RNase treatment at 37 degrees Celsius for one hour. This stepmay optionally be removed (as in Process II described in Example 2below) by utilizing the preferred alkaline lysis and high salt agentsdescribed herein.

Step 5 involves filtration and dilution with 2 volumes of WFI. These arestandard techniques that may easily be modified by those skilled in theart.

Step 6 involves the use of a Q Sepharose HP column to remove protein,and some RNA and chromosomal DNA. Other Q resins may also work, or aFractogel TMAE resin may be used as in Process II below.

Step 7, involves the use of DEAE 650-S to remove RNA.

Step 8 uses Phenyl 650-S to remove chromosomal DNA, endotoxin, denaturedplasmid, and some RNA. Other phenyl resins and 8 carbon octyl resins mayalso be used.

Steps 9 and 10 are ultrafiltration/diafiltration and sterile filtrationto yield the final product. These are common procedures that may beinsubstantially modified by those skilled in the art without departingfrom the scope of the invention.

Example 2 Process II

The steps of Process II are outlined in FIG. 3. Step (a) is fermentationof plasmid containing E. coli, cell harvesting, e.g. by centrifugationand washing and resuspending cells as in step 1 of Process I.

Step (b) is alkaline lysis (e.g., 1% SDS and 0.2N NaOH) and high saltprecipitation to extract plasmid and remove chromosomal DNA and some RNAand protein. The high salt precipitation may involve adding 1M potassiumacetate and 7M ammonium acetate (concentrations of both may be varied,with higher concentrations of ammonium acetate being better for RNAprecipitation) and holding the temperature at 4 degrees Celsius forapproximately 6 to 12 hours to precipitate RNA (times and temperaturesmay be varied; lower temperatures and longer times improve precipitationand incubation at 4 degrees Celsius provides a higher percentage ofprecipitation). The ability to precipitate RNA during neutralization ofthe alkaline lysate helps eliminate the need for RNase. The acetatesolution does not need to be pH adjusted and the pH is approximately 8.Conventional techniques using 3M potassium acetate at pH 5 aredramatically less effective at RNA precipitation and provide greaterrisk of DNA damage.

Step (c) is centrifugation/filtration and 1.5 volume dilution with waterfor irrigation (WFI).

Step (d) involves the use of, for example, Fractogel EMD TMAE 650(s), astrong anionic change resin, to remove residual protein and some RNA andchromosomal DNA and enrich the supercoiled form of the plasmid DNA. Thisresin is capable of separating nicked/relaxed plasmid from supercoiledplasmid with a preparative, low pressure, step elution. Variations inparticle size and chemistry may be acceptable. The next procedure is towash the nicked and/or relaxed circular plasmid, as well as residualRNA, off of the resin with ˜0.6M NaCl (10 mM Tris, 1 mM EDTA, pH 8.0).The supercoiled plasmid thus remains bound to resin. Other salts mayalso be suitable at an appropriate concentration (depending on thesalt). This is followed by eluting the covalently closed supercoiledplasmid off of the resin with about 1.9M ammonium sulfate (A.S.).Elution with concentrated A.S. allows eluate to flow through the nextcolumn. Elution with ≧0.7M NaCl is also suitable, but in that case theeluate must be adjusted to ˜1.6 M.A.S. prior to the next column. Othersalts may also be suitable at an appropriate concentration (depending onthe salt).

Step (e) involves the use of Octyl Sepharose 4FF for removal ofchromosomal DNA, RNA, endotoxin, and denatured plasmid. This involvespassing the TMAE pool through an Octyl Sepharose 4 Fast Flow (Pharmacia#17-0946) resin and collecting the flow through fraction. Otherhydrophobic interaction (HIC) resins may also be used (e.g. PharmaciaPhenyl Sepharose, Pharmacia Butyl Sepharose, similar resins from othervendors). At ˜1.6M ammonium sulfate, plasmid flows through and residualRNA, endotoxins, and E. coli DNA bind. Alternatively, one can bind theTMAE pool in the presence of ˜2.0M ammonium sulfate, which allowsrelaxed and/or nicked circular plasmid to flow through, while thesupercoiled plasmid binds; the supercoiled can then be eluted with ≦1.6Mammonium sulfate. Variations in resin particle size, etc., may beacceptable and some alternative salts are also suitable at appropriateconcentrations.

Steps (f) and (g) are ultrafiltration/diafiltration followed by sterilefiltration to yield final product.

Some of the advantages of Process II are listed below: (1) Canenrich/resolve for supercoiled plasmid DNA; (2) Plasmid DNA is neverprecipitated; (3) No linear gradients necessary; (4) No organic solventsused; (5) In preferred embodiments in Process I, 1.62 volumes of lysissolution (neutralizing) is used whereas in preferred embodiments ofProcess II, 1 volume of lysis solution (high salt 1M potassium acetateand 7M NH₄OAc) is used; (6) No RNase required; (7) Significantly reducesendotoxin levels; (8) DNA is not exposed to either acid pH or elevatedtemperatures; (9) Scaleable for Industrial Quantities; and (10) Highplasmid DNA yields (60-90%) and high purity (90-95%).

Example 3 Unit Operations of the Invention

The unit operations in this invention are in-line static mixers, flowthrough lysis coils, and a chilled jacketed tank.

The cell paste, either from direct isolation from fermentation or thawedif frozen, was resuspended in TE buffer (50 mM Tris and 10 mM EDTA) bycirculating the cell-buffer mixture through the in-line static mixer ina pump-around mode. The flow rate was specifically determined to becapable of effectively resuspending cells without breaking them.

The resuspended cells were mixed with the lysis solution (200 mM NaOHand 1% SDS) by flowing through the in-line static mixer, then enteringthe lysis coils for continuous lysis at a constant rate. The optimalflow rate to deliver cell-lysis solution mixture through the staticmixer and lysis coils was determined to maximize the flow rate, minimizethe shear force, and generate homogeneous mixture and desired moleculeresidence time.

The lysate was mixed with cold neutralization solution (1.85M KOAc,1.16N NH₄OAc, and 1.15M NaCl pH 5.5 Process I buffer solution) todenature and precipitate high molecular weight host chromosomal DNA,host proteins, and host RNA. The mixing of neutralization solution andlysate is either accomplished by flowing through the in-line staticmixer in a continuous mode, or by agitating the low speed impeller mixerin a batch mode.

After mixing, the acidified lysate will be retained in the chilledjacketed tank for 20-30 minutes to complete the precipitation anddenaturation.

Example 4 Scale-Up Test

The scale-up test was started from transferring thawed cells and TEbuffer into the conical tank. The cell-TE mixture was then pumpedthrough the in-line static mixer to the optimized flow rate. Aftercompleting cell resuspension, cells and lysis solution were pumpedthrough another in-line static mixer simultaneously for mixing. At thesame time of mixing cells and lysis solution, cells remained in theconical tank were still circulated through the resuspension-in-linestatic mixer to ensure the homogeneity of cell pool to maximize theeffectiveness of alkaline lysis. Lysate continuously flowed through thelysis coils at a fixed residence time.

The effluent from the lysis coils was directed to a chilled jacketedtank containing a high salt neutralizing solution and equipped with animpeller stirrer for mixing for mixing. However, the preferred methodwould be to mix the highly viscous alkaline cell lysis solution with themuch lower viscosity high salt solution using an in-line static mixer tominimize the shearing of the chromosomal DNA. This would allow forinstantaneous mixing of the two solutions in a gentle manner to form thedenatured chromosomal DNA/denatured protein/sodium dodecyl sulphatecomplex. Afterwards the neutralized precipitated lysate would bedirected to a cold jacketed holding tank.

Example 5 Isolation of Plasmid

1.5 kg (wet weight) of frozen recombinant E. coli cells harboring aplasmid was thawed and TE buffer was added to make a total volume to10.5 L. The mixture of cell paste and TE buffer was circulating throughthe in-line mixers at 3 L/min for 5-10 minutes at room temperature toobtain homogeneous cell concentration. The resuspended cells and roomtemperature lysis solution were pumped to the 1″ in-line static mixersimultaneously at an overall flow rate of 1.0 L/min. The mixture ofcells and lysis solution continuously flowed through the lysis coils at1.0 L/min.

The lysate exiting from the lysis coils directly into the jacketed tankwhich contained 17 L cold neutralization buffer. The lysate and coldneutralization solution were gently mixed by an impeller mixer to avoidshearing the host chromosomal DNA. The acidified lysate was kept in thechilled-jacketed tank for about 30 minutes to completely precipitate thehost chromosomal DNA, host proteins, and host RNA.

After completing the precipitation, plasmid was isolated and purified.The host chromosomal DNA level in the final product was assayed at 0.4%.

Example 6 Reproducibility of the Alkaline Lysis Process

1.5 kg (wet weight) of frozen recombinant E. coli cells harboring aplasmid was used to examine the reproducibility of the invented process.All buffers, devices, and procedures were adopted to mimic Example 5.The isolation and purification procedures were also the same as thatused in Example 5. The host chromosomal DNA level in the final productwas assayed at 0.12%.

Example 7 Variability of Final Product Quality

Without Use of With Use of Device Device Number of Lots Used forComparison 37 7 Average Chromosomal DNA, % 2.83 0.63 Standard Deviation2.68 0.64

The above showed that the use of the device is superior in regard toboth final product quality (chromosomal DNA level) and processperformance consistency.

Example 8 Purification of Plasmid DNA Containing the α1-Antityrpsin Gene

Previous purification of an α₁-antitrypsin plasmid prep had been done bythe process 1 protocol (FIG. 1) in which the H.I.C. step had not yetbeen incorporated. Thus the purification process ended after the DEAEcolumn step and the resulting plasmid pool was concentrated by standardethanol precipitation. The final product of α₁-antitrypsin plasmidprepared by the above method still contained unacceptable high levels ofchromosomal DNA (bacterial), RNA, denatured plasmid, and endotoxins.

Therefore a phenyl hydrophobic interaction chromatography (HIC fromTosoHaas) step was added to the process 1 protocol following the DEAEchromatography column in order to reduce the contaminants to a muchlower level and thereby increasing the purity of the plasmid DNA to alevel ≧95%.

Approximately 315 grams of DH-5α E. coli cells (wet weight) containingthe α₁-antitrypsin plasmid was exposed to alkaline lysis for 5 minutesand the denatured chromosomal DNA/protein/sodium dodecylsulfatecomplexes were precipitated by addition of a salt solution containing1.85M potassium acetate, 1.15M sodium chloride, 1.16M ammonium acetateat pH 5.5. After centrifugation, filtration, and RNAse treatment theresulting supernatant (8L) contained ˜35 μg plasmid DNA/mL.

Following anion exchange chromatography of the lysate on both QSepharose 4FF (Pharmacia) and DEAE 650-S (TosoHaas) which removesprotein, RNA and the bulk of chromosomal DNA, the plasmid pool was thenpumped onto a phenyl 650-S (TosoHaas) HIC resin.

The phenyl 650-S resin (100 mL) was packed into a 5 cm diameter columnto a height of 5 cm. The column was equilibrated with approximately 500mL of 1.75M (NH₄)₂SO₄ at 20 mL/minute. Then 350 mL of the DEAE elutionpool containing the α₁-antitrypsin plasmid was diluted with an equalvolume of 3.5M (NH₄)₂SO₄ which results in a final concentration of 1.75M(NH₄)₂SO₄. The entire 700 mL of plasmid DNA in 1.75M (NH₄)₂SO₄ waspumped through the HIC column and the flow through peak which containsthe α₁-antitrypsin plasmid was saved for further processing byultrafiltration and diafiltration (UF/DF). The bound RNA, chromosomalDNA, denatured plasmid and endotoxins were eluted in a Milli-Q waterwash followed by a 70% ethanol (v/v) wash.

A Pellicon-2 system (Millipore) containing a 50 kD casseter (Millipore)was used to UF/DF the HIC flow through containing the α₁-antitrypsinplasmid DNA in 1.75M (NH₄)₂SO₄. The final product (α₁-antitrypsinplasmid) was concentrated 7.5-fold by UF, and diafiltered against 10diavolumes of Milli-Q water.

Analysis of the final product, α₁-antitrypsin plasmid, indicated thepreparation 95% plasmid DNA, <5%RNA, 0.05% E. coli DNA, <0.06% protein,<0.2 Eu//mg of plasmid DNA for endotoxins, and complete removal ofdenatured plasmid DNA. This represents a reduction by the HIC resin inbacterial chromosomal DNA of between 10 and 100-fold from what typicallycontaminates plasmid preps after the DEAE anion chromatography step. Inaddition, RNA was reduced between 1 and 2-fold, endotoxin levels werelowered at least 250-fold, both protein and denatured plasmid levelshave been reduced to essentially zero.

It should also be pointed out that most HIC resins will bind supercoiledDNA at ≧2M (NH₄)₂SO₄ while the open circle form will not bind underthese conditions. This allows for a significant enrichment of thesupercoiled plasmid DNA in the final product. Thus HIC resins inconjunction with anionic-exchange resins offer the ability atlarge-scale to remove a substantial amount of contaminants whichcopurify with the plasmid DNA on ion-exchange resins. In addition, HICresins can be used to selectively purify supercoiled plasmid DNA.

Example 9 A Hydrolysis Assay

This assay is used to quantify the amount of RNA present in a solutionof plasmid DNA.

Standard Curve

(1) Use E. coli rRNA as standards from Sigma; ribonucleic acid,ribosomal from E. coli, Strain W; Cat. No. R-7628). Make up rRNA toconcentrations of 0, 10, 20, 30, 40 and 50 μgs/ml by diluting theresuspended RNA in water.

(2) Run 367 μl of each standard through RNA hydrolysis procedure (seebelow).

(3) Plot standards with concentration versus A260 and curve fit with alinear fit, forcing the curve through zero.

(4) Use the slope from the curve to calculate sample RNA concentrations.

Hydrolysis Procedure

(1) Take 467 μls of sample. If necessary, dilute before taking 467 μlsso that RNA is less than 50 μgs/ml. Put samples in screw cap tubes sothat tubes do not pop open during heating.

(2) Add 33 μl 1.5M KOH to bring volume to 0.5 mls.

(3) Heat at 110° C. for 1 hour. The heat and alkali will hydrolyze theRNA.

(4) Turn off heat block after one hour, leaving the samples in theblock, and allow samples to cool to less than 100° C. (about 10minutes).

(5) Remove from heat block and add 32 μls of 10% HCl.

(6) Add 968 μls 100% ethanol to bring volume to 1.5 mls. The absoluteethanol will precipitate the DNA in the sample.

(7) Place the samples at −20° C. for at least 20 minutes.

(8) Centrifuge samples at 4° C. for at least 15 minutes at 14,000 rpm.

(9) Blank UV spectrophotometer with WFI, water for injection or MilliQwater. Measure A260 and A232 of supernatants.

(10) Use A260 to calculate RNA concentration with the standard curveand, if necessary, use A232 to adjust for protein.

One skilled in the art would readily appreciate that the presentinvention is well adapted to carry out the objects and obtain the endsand advantages mentioned, as well as those inherent therein. Themethods, procedures, treatments, molecules, specific compounds describedherein are presently representative of preferred embodiments areexemplary and are not intended as limitations on the scope of theinvention. Changes therein and other uses will occur to those skilled inthe art which are encompassed within the spirit of the invention aredefined by the scope of the claims. It will be readily apparent to oneskilled in the art that varying substitutions and modifications may bemade to the invention disclosed herein without departing from the scopeand spirit of the invention.

All patents and publications mentioned in the specification areindicative of the levels of those skilled in the art to which theinvention pertains. All patents and publications are herein incorporatedby reference to the same extent as if each individual publication wasspecifically and individually indicated to be incorporated by reference.

1. A method of purifying plasmid DNA from a plasmid pool comprisingcontacting the plasmid pool with a hydrophobic interactionchromatography (HIC) resin in an aqueous solution, whereby the plasmidDNA is separated from one or more contaminants in the plasmid poolsolution.
 2. The method of claim 1, wherein the HIC resin is moderatelyhydrophobic and comprises alkyl chain ligands.
 3. The method of claim 2,wherein the alkyl chain ligands are selected from the group consistingof: octyl, phenyl, and butyl alkyl chain ligands.
 4. The method of claim1, wherein the resin comprises agarose.
 5. The method of claim 4,wherein the HIC resin comprising agarose is selected from the groupconsisting of: octyl sepharose, phenyl sepharose, and butyl sepharose.6. The method of claim 1, wherein the HIC resin is selected from thegroup consisting of: octyl sepharose, phenyl sepharose, butyl sepharoseand phenyl 650-S.
 7. The process of claim 1, wherein the aqueoussolution comprises a salt at a concentration of at least 1.6 molar. 8.The method of claim 1, wherein the salt is a multivalent salt selectedfrom the group consisting of: ammonium salts, sodium salts, potassiumsalts, and combinations thereof.
 9. The method of claim 1, wherein themultivalent salt is selected from the group consisting of: ammoniumsulfate, sodium citrate and potassium phosphate salts.
 10. A process forisolating plasmid DNA from bacterial cells, comprising the steps of: (1)fermenting and harvesting the cells; (2) exposing the cells to alkalinelysis and precipitation thereby forming a plasmid pool; (3) purifyingthe plasmid pool by hydrophobic interaction chromatography (HIC) in anaqueous solution containing a salt at concentration of at least 1.6 M,whereby the plasmid DNA is differentially separated from one or morecontaminants of the plasmid pool.
 11. The process of claim 10, whereinthe HIC is performed using a HIC resin that is moderately hydrophobicdue to alkyl chains as ligands.
 12. The process of claim 11, wherein thealkyl chain ligands are selected from the group consisting of: octyl,phenyl, and butyl alkyl chain ligands.
 13. The process of claim 10,wherein the salt is selected from the group consisting of: ammonium,sodium, and potassium salts, and combinations thereof.
 14. The processof claim 10, further comprising contacting the plasmid pool with ananion exchange chromatography step.
 15. The method of claim 10, wherethe plasmid DNA is not precipitated, exposed to elevated temperatures oracidic pH, and wherein the method uses no organic solvents.
 16. Themethod of claim 11, wherein the HIC is performed at a salt concentrationwherein the plasmid DNA is collected in a flow through fraction andresidual RNA, endotoxins, and E. Coli DNA remain bound to the resin. 17.The method of claim 11, wherein the HIC is performed at a saltconcentration wherein a supercoiled plasmid DNA population is bound toresin and a relaxed and/or nicked circular plasmid population is allowedto flow through the resin.
 18. A method of separating supercoiledplasmid DNA from a mixture of supercoiled plasmid DNA and relaxed opencircle plasmid DNA and, optionally, at least one host cell impurity,comprising the following steps: (a) forming a solution by adding a saltto the mixture of supercoiled plasmid DNA and relaxed plasmid DNA and,when present, said at least one host cell impurity; (b) contacting thesolution with a chromatography support comprising a hydrophobic ligandat a salt concentration where the supercoiled plasmid DNA and therelaxed plasmid DNA differentially interact with the hydrophobic ligandleading to a resolution of forms; and (c) altering the saltconcentration to remove supercoiled plasmid DNA from the chromatographysupport under conditions where, when present, said at least one hostcell impurity is retained on the support.
 19. The method of claim 18,wherein the impurity is selected from the group consisting of:chromosomal DNA, RNA, protein, endotoxin, and combinations thereof. 20.The method of claim 18, wherein said hydrophobic ligand is an alkylchain ligand selected from the group consisting of: octyl, phenyl, andbutyl alkyl chain ligands.
 21. The process of claim 18, wherein the saltconcentration is at least 1.6 molar.